Piezoelectric Aluminum Nitride Thin Films Research Articles

Modeling and Electrical Characterization of a Bilayer Pt/AlN/Sapphire One Port Resonator for Sensor Applications

This paper presents a two-dimensional FEM (Finite Element Method) modeling and simulation of a surface acoustic wave (SAW) resonator based on a layered Pt/AlN/Sapphire structure. Such structure that exploits the electromechanical coupling of piezoelectric film is of high interest for harsh environments. By harsh environment we mean any environment that could hinder the operation of the device. Hardness can come from a variety of sources, and examples include the following: High pressure, High temperature, Shock/high vibration, Radiation, Harsh chemicals, etc. As part of this work, we are looking for high temperature sensor applications and only operating drifts due to temperature will be studied. SAW resonator is made from piezoelectric thin film Aluminum Nitride (AlN) layer on Sapphire substrate. Modal analysis is used to determine the eigen mode and the eigenfrequency of the system and the study of the frequency domain is used to determine the response of the model under influence of a harmonic excitation for one or more frequencies. In the FEM modeling, various parameters of the surface waves in the films, such as the surface velocity, the displacement of the piezoelectric thin film, the electrical potential, the electromechanical coefficient (k2), and the quality factor (Q) were studied. A comparative study between modeled and experimental curves showed a good agreement and allowed to validate our simulation method. Finally, a FEM study of the influence of normalized thickness of AlN thin film on resonator performances was carried out and compared with theorical results of literature.

2D Scanning Micromirror with Large Scan Angle and Monolithically Integrated Angle Sensors Based on Piezoelectric Thin Film Aluminum Nitride.

A 2D scanning micromirror with piezoelectric thin film aluminum nitride (AlN), separately used as actuator and sensor material, is presented. For endoscopic applications, such as fluorescence microscopy, the devices have a mirror plate diameter of 0.7 mm with a 4 mm2 chip footprint. After an initial design optimization procedure, two micromirror designs were realized. Different spring parameters for x- and y-tilt were chosen to generate spiral (Design 1) or Lissajous (Design 2) scan patterns. An additional layout, with integrated tilt angle sensors, was introduced (Design 1-S) to enable a closed-loop control. The micromirror devices were monolithically fabricated in 150 mm silicon-on-insulator (SOI) technology. Si (111) was used as the device silicon layer to support a high C-axis oriented growth of AlN. The fabricated micromirror devices were characterized in terms of their scanning and sensor characteristics in air. A scan angle of 91.2° was reached for Design 1 at 13 834 Hz and 50 V. For Design 2 a scan angle of 92.4° at 12 060 Hz, and 123.9° at 13 145 Hz, was reached at 50 V for the x- and y-axis, respectively. The desired 2D scan patterns were successfully generated. A sensor angle sensitivity of 1.9 pC/° was achieved.

Sensitivity and Directivity Analysis of Piezoelectric Ultrasonic Cantilever-Based MEMS Hydrophone for Underwater Applications

In this paper, we report on the characterization of the sensitivity and the directionality of a novel ultrasonic hydrophone fabricated by micro-electro-mechanical systems (MEMS) process, using aluminum nitride (AlN) thin film as piezoelectric functional layer and exploiting a stress-driven design. Hydrophone structure and fabrication consist of four piezoelectric cantilevers in cross configuration, whose first resonant frequency mode in water is designed between 20 kHz and 200 kHz. The MEMS fabricated structures exploit 1 µm and 2 µm thick piezoelectric AlN thin film embedded between two molybdenum electrodes grown by DC magnetron sputtering on silicon (Si) wafer. The 200 nm thick molybdenum electrodes thin layers add a stress-gradient through cantilever thickness, leading to an out-of-plane cantilever bending. A water resistant parylene conformal coating of 1 µm was deposited on each cantilever for waterproof operation. AlN upward bent cantilevers show maximum sensitivity up to −163 dB. The cross configuration of four stress-driven piezoelectric cantilevers, combined with an opportune algorithm for processing all data sensors, permits a finer directionality response of this hydrophone.

Piezoelectric Scanning Micromirror With Built-In Sensors Based on Thin Film Aluminum Nitride

A micromirror with piezoelectric thin film aluminum nitride (AIN) as transducer material for actuation and detection is presented. Four sensor elements are integrated for closed-loop monitoring of the resonant driven microscanner with an entire footprint of 6 mm2. The sensor and actuator elements are monolithically fabricated in 150 mm SOI technology. The micromirror is characterized in terms of its scanning and sensor characteristics. A large optical scan angle of 137.9° (34.5° mechanical tilt angle corresponding) at 3385 Hz and 20 V actuation voltage is achieved. The median resonant voltage sensitivity is 4.7 °/V. For the sensor characterization, a cross-talk compensation is introduced, which significantly improves the signal quality and thus enables an accurate charge determination. The sensor signal increases linearly to the deflection. A dynamic sensor angle sensitivity of 48.4 fC/° is reached. Possible applications of the micromirror are consumer electronics, such as LIDAR or medical applications, such as fluorescence microscopy.

High-Gauge Factor Strain Sensor Based on Piezoelectric Aluminum Nitride Coupled to MOSFET

The development of high-sensitive and economical strain sensors is crucial for precise and efficient measurement of ultra-small strain in industrial applications. This paper reports a microfabricated strain sensor based on aluminum nitride (AlN) combined with a metal-oxide-semiconductor field effect transistor (MOSFET). A high-quality AlN thin film works as a sensing element, which is connected electrically to the gate electrode of the MOSFET for the amplification of piezoelectric response of the AlN thin film. The pseudo gauge factor (GF) of the MOSFET is evaluated from the source-drain resistance change under the application of stress to the piezoelectric AlN thin film. A GF of as high as 1340 is obtained, which is seven times larger than the best GF demonstrated for the state-of-the-art silicon piezoresistive strain sensor. The exponential change of drain-source current with gate voltage in subthreshold region of the MOSFET offers a large enhancement of the sensitivity of such sensors. The developed strain sensor has various applications in strain and stress measurements in an ultra-small strain range.

Effect of AlN seed layer on crystallographic characterization of piezoelectric AlN

Ultrathin aluminum nitride (AlN) films are of great interest for integration into nanoelectromechanical systems for actuation and sensing. Given the direct relationship between crystallographic texture and piezoelectric (PZE) response, x-ray diffraction has become an important metrology step. However, signals from layers deposited below the PZE AlN thin film may skew the crystallographic analysis and give misleading results. In this work, the authors compare the use of a Ti or AlN seed layer on the crystallographic quality of PZE AlN. The authors also analyze the influence of several AlN seed layer thicknesses on the rocking curve FWHM of PZE AlN and demonstrate a larger effect of the AlN seed layer on the θ-2θ AlN <0002> crystallographic peak for increasing AlN seed layer thickness.

Flexible Piezoelectric Energy-Harvesting Exploiting Biocompatible AlN Thin Films Grown onto Spin-Coated Polyimide Layers

The increasing demand of piezoelectric energy harvesters for wearable and implantable applications requires biocompatible materials and careful structural device design, paying special attention to the conformability characteristics, properly tailored to scavenge continuously electrical energy even from the tiniest body movements. This paper provides a comprehensive study on a flexible and biocompatible aluminum nitride (AlN) energy harvester based on a new alternative fabrication approach, exploiting a thin polyimide (PI) substrate, prepared by spin coating of precursors solution. This strategy allows manufacturing substrates with adjustable thickness to meet conformability requirements. The device is based on a piezoelectric AlN thin film, sputtered directly onto the soft PI substrate, without poling/annealing processes and patterned by simple and low cost microfabrication technologies. AlN active layer, grown on soft substrate, exhibits good morphological and structural properties with roughness root m...

Surface Morphology and Microstructure of Pulsed DC Magnetron Sputtered Piezoelectric AlN and AlScN Thin Films

Reactive pulsed DC magnetron co‐sputtering is used to grow piezoelectric aluminum nitride (AlN) and aluminum scandium nitride (AlScN) thin films on Si(001) substrates. By using grazing incidence X‐ray diffraction (GIXRD), scanning electron microscopy (SEM), and piezoresponse force microscopy (PFM), we investigate how the microstructure and the presence of misoriented grains affect the piezoelectric properties of the material. N2 concentration and target‐to‐substrate distance are finely tuned to achieve thin films without misoriented grains, resulting in Al0.87Sc0.13N thin films with low roughness, high degree of c‐axis orientation, homogenous polarity, and piezoelectric coefficient d33 = −12.3 pC/N.

Quantum dot emission modulation using piezoelectric photonic crystal MEMS resonators.

Quantum dots (QDs) integration into photonic devices requires varied approaches to control and modulate their emission. We demonstrate voltage-tunable PC structures with integrated QDs over suspended piezoelectric aluminum nitride thin film resonators that modulate PC enhancement at MHz frequencies. When the piezoelectric device is actuated at its resonant mechanical frequency, the extracted QD emission direction is likewise modulated via the optical resonant frequency of the PC. Modulation uses nanometer-scale mechanical displacements, offering the potential for greater switching speed and improved mechanical robustness that is not subject to the effects of stiction with a scalable fabrication approach.

Low Temperature Reactive Sputtering of Thin Aluminum Nitride Films on Metallic Nanocomposites.

Piezoelectric aluminum nitride thin films were deposited on aluminum-molybdenum (AlMo) metallic nanocomposites using reactive DC sputtering at room temperature. The effect of sputtering parameters on film properties was assessed. A comparative study between AlN grown on AlMo and pure aluminum showed an equivalent (002) crystallographic texture. The piezoelectric coefficients were measured to be 0.5±0.1 C m-2 and 0.9±0.1 C m-2, for AlN deposited on Al/0.32Mo and pure Al, respectively. Films grown onto Al/0.32Mo however featured improved surface roughness. Roughness values were measured to be 1.3nm and 5.4 nm for AlN films grown on AlMo and on Al, respectively. In turn, the dielectric constant was measured to be 8.9±0.7 for AlN deposited on Al/0.32Mo seed layer, and 8.7±0.7 for AlN deposited on aluminum; thus, equivalent within experimental error. Compatibility of this room temperature process with the lift-off patterning of the deposited AlN is also reported.

Flexible structured high-frequency film bulk acoustic resonator for flexible wireless electronics

Flexible electronics have inspired many novel and very important applications in recent years and various flexible electronic devices such as diodes, transistors, circuits, sensors, and radiofrequency (RF) passive devices including antennas and inductors have been reported. However, the lack of a high-performance RF resonator is one of the key bottlenecks to implement flexible wireless electronics. In this study, for the first time, a novel ultra-flexible structured film bulk acoustic resonator (FBAR) is proposed. The flexible FBAR is fabricated on a flexible polyimide substrate using piezoelectric thin film aluminum nitride (AlN) for acoustic wave excitation. Both the shear wave and longitudinal wave can be excited under the surface interdigital electrodes configuration we proposed. In the case of the thickness extension mode, a flexible resonator with a working frequency as high as of 5.2325 GHz has been realized. The resonators stay fully functional under bending status and after repeated bending and re-flattening operations. This flexible high-frequency resonator will serve as a key building block for the future flexible wireless electronics, greatly expanding the application scope of flexible electronics.

High Temperature Microelectromechanical Systems Using Piezoelectric Aluminum Nitride

We report on the efforts at Sandia National Laboratories to develop high temperature capable microelectromechanical systems (MEMS). MEMS transducers are pervasive in today's culture, with examples found in cell phones, automobiles, gaming consoles, and televisions. There is currently a need for MEMS transducers that can operate in more harsh environments, such as automobile engines, gas turbines, nuclear and coal power plants, and petroleum and geothermal well drilling. Our development focuses on the coupling of silicon carbide (SiC) and aluminum nitride (AlN) thin films on SiC wafers to form a MEMS material set capable of temperatures beyond 1000°C. SiC is recognized as a promising material for high temperature capable MEMS transducers and electronics because it has the highest mechanical strength of semiconductors with the exception of diamond and its upper temperature limit exceeds 2500°C, where it sublimates rather than melts. Most transduction schemes in SiC are focused on measuring changes in capacitance or resistance, which require biasing or modulation schemes that can withstand elevated temperatures. Instead, we are coupling temperature hardened, micro-scale SiC mechanical components with piezoelectric AlN thin films. AlN is a non-ferroelectric piezoelectric material, enabling piezoelectric transduction at temperatures exceeding 1000°C. AlN is a favorable MEMS material due to its high thermal, electrical, and mechanical strength. It is also closely matched to SiC for coefficient of thermal expansion.

Pulsed-DC sputtering of molybdenum bottom electrode and piezoelectric aluminum nitride films for bulk acoustic resonator applications

Various sputtering conditions were employed to explore the feasibility of depositing a suitably textured layer of molybdenum film, as a bottom-electrode of a film bulk acoustic resonator, on a silicon substrate. A fully (110)-textured Mo film, with its full width at half maximum (FWHM) of rocking curve as low as 1.1°, could be made when a 25-nm thick primer layer of aluminum nitride (AlN) film was pre-deposited between Si and Mo. In turn, the degree of the (0002) texture of a subsequently deposited AlN piezoelectric film, about 1.4 μm in thickness, was found to be largely decided by the degree of the (110) texture of the Mo film beneath it. The residual stress of this AlN piezoelectric film also varied virtually according to the texture quality of the underlying Mo film. The optimal process condition resulted in a piezoelectric AlN thin film having a (0002) FWHM as low as 0.98°, and a slightly compressive residual stress of 439 MPa at the same time.

A contour-mode film bulk acoustic resonator of high quality factor in a liquid environment for biosensing applications

This letter reports an acoustic resonator of high quality factors (Qs) operating in liquid media. The film bulk acoustic resonator (FBAR) is made of a ring-shaped piezoelectric aluminum nitride thin film, and is excited in a contour mode. By having a low motional resistance upon coupling with liquids, the contour mode FBAR achieved Qs up to 189, more than 12× over the state-of-the-art FBARs in liquids. The resonator was characterized by an aptamer—thrombin binding pair for a biosensor and showed a mass resolution of 1.78 ng/cm2.

Simulation and laser vibrometry characterization of piezoelectric AlN thin films

In this paper, the electric field induced deformations of sputter-deposited piezoelectric aluminum nitride thin films sandwiched between electrodes on top of a silicon substrate are studied by numerical calculations and scanning laser interferometric measurements. In our calculations based on the finite element method, the results show the displacement of the top and bottom surfaces of both the thin film and the substrate, for either a free or a perfectly clamped structure. The confirmation that the bottom surface of the film is deformed reveals the limitations of techniques that only access the top surface, as well as the double-beam interferometric configuration, under specific conditions. In addition, the simulations demonstrate the dependence of the displacements on the size of the upper electrode and the contribution of the transverse piezoelectric coefficient d31 to the features of the displacement profiles. A laser scanning vibrometry technique was used to measure deformations on the top surface with subpicometer vertical resolution. By comparing the calculated and the experimental displacement profiles, an advanced approach is discussed to obtain accurate quantitative information of both coefficients d31 and d33.

Combination of e-Beam Lithography and of High Velocity AlN/Diamond-Layered Structure for SAW Filters in X Band

In this work, we report on the fabrication results of surface acoustic wave (SAW) devices operating at frequencies up to 8 GHz. In previous work, we have shown that high acoustic velocities (9 to 12 km/s) are obtained from the layered AIN/diamond structure. The interdigital transducers (IDTs) made of aluminium with resolutions up to 250 nm were successfully patterned on AIN/diamond-layered structures with an adapted technological process. The uniformity and periodicity of IDTs were confirmed by field emission scanning electron microscopy and atomic force microscopy analyses. A highly oriented (002) piezoelectric aluminum nitride thin film was deposited on the nucleation side of the CVD diamond by magnetron sputtering technique. The X-ray diffraction effectuated on the AIN/diamond-layered structure exhibits high intensity peaks related to the (002) AIN and (111) diamond orientations. According to the calculated dispersion curves of velocity and the electromechanical coupling coefficient (K2), the AIN layer thickness was chosen in order to combine high velocity and high K2. Experimental data extracted from the fabricated SAW devices match with theoretical values quite well.

Study of structural and microstructural properties of AlN films deposited on silicon and quartz substrates for surface acoustic wave devices

Piezoelectric aluminum nitride thin films were deposited by reactive dc magnetron sputtering under various experimental conditions on a silicon (100) substrates. These films structural and microstructural properties were studied using x-ray diffraction (XRD), selected area electron diffraction (SAED), and transmission electron microscopy (TEM) in order to determine the optimum growth conditions that improve the piezoelectric coupling. A highly c-axis oriented AlN film (002) was identified from XRD and SAED. An average grain column size of about 40 nm was determined from TEM. The root-mean-square surface roughness measured using atomic-force microscopy was less than 7 nm. The optimum experimental parameters found for AlN deposition on Si(100) substrates were then extrapolated to its deposition on a ST-cut quartz substrate for the investigation of the influence of substrates on the surface acoustic wave (SAW) propagation. The AlN/Si and AlN/ST-quartz SAW devices were formed using the photolithography process. Their operating characteristics, phase velocity, temperature coefficient of frequency, and electromechanical coefficient (K2) were measured and analyzed. Experimental results show that the generalized SAW (GSAW) as well as the pseudosurface acoustic wave were excited in this structure. Their phase velocities are, respectively, 3245 m/s and 5117 m/s. The SAW device realized from the AlN/ST-cut quartz structure exhibits higher phase velocity and higher K2 compared to those formed on the ST-quartz substrate.

Process Development and Integration of Piezoelectric Aluminum Nitride Thin-Film for RF MEMS Applications

ABSTRACTThe aluminum nitride (AlN) process was successfully developed for as high frequency thin-film for bulk acoustic resonator applications. Columnar AlN was deposited on p-type (100) silicon substrate at 300–500°C using pulse direct current (DC) sputtering technique. Argon to nitrogen ratio was optimized to achieve stoichiometry at higher deposition rates of about 1000 Å/min at 1–4 kW DC power. The XRD of as-deposited and annealed AlN film shows preferred (002) orientations. The dry etching process was developed and etching rate about 3200 Å/min was achieved using Cl based chemistry. Film composition analysis was done using AES technique and stoichiometry was achieved.